Wollastonite is the calcium end member of the pyroxenoids, having a formula unit composition of CaSiO3. At high temperatures it forms an extensive solid solution with FeSiO3 and MnSiO3 (rhodonite); at low temperatures two intermediate members, hedenbergite CaFe(SiO3)2 and bustamite (Mn,Ca,Fe)SiO3 form.

Pyroxenoids are similar to pyroxenes with the exception that the silica chains have longer repeat patterns due to twisting or other types of alteration of the silica chain. The repeat distance in wollastonite is 3 silica units; that of rhodonite is 5, and some others are even longer.

Distribution

Wollastonite commonly occurs in contact metamorphic zones in limestone. It can form from calcite and quartz at temperatures above 600 to 700 ° C. It commonly occurs with calcite, tremolite, diopside, anorthite, and a number of rare calcium-magnesium silicates.

Ohashi, Y., and L.W. Finger. 1978. The role of octahedral cations in pyroxenoid crystal chemistry. I. Bustamite, wollastonite, and the pectolite-schizolite-serandite series sample Fe-WO. American Mineralogist 63:274-288.